Field of the Invention
This invention relates to a navigation system for vehicles, and more particularly to a navigation system capable of providing a route to destination display and a vocal guide message for driving.
Description of the Related Art
There is a stand-alone type navigation system known as a position measuring device used in vehicles such as automobiles, airplanes or ships. Stand-alone type navigation systems detect the two-dimensional displacement (vector) from azimuth data obtained by an azimuth sensor and velocity data obtained by a velocity sensor, and add the two-dimensional displacement to data of reference points to calculate current position of vehicle. In a case of automobiles, a predetermined number of pulses are generated for every rotation of the drive shaft of the automobile, and the travel distance between the reference point and current position is calculated on the basis of the total number of pulses thus generated. Then, the exact travel distance is obtained by multiplying the travel distance by a distance correction coefficient.
As a position measuring device utilizing space satellites, GPS (Global Positioning System) navigation system has been developed. The GPS navigation system generally receives electric waves from more than three GPS satellites, and calculates pseudo distance data including offset time between each of GPS satellites and electric wave receiving point and current position data of electric wave receiving point on the basis of the position data of the GPS satellites. Then, the GPS navigation system reads out map data around the calculated current position, using information processor unit, from a storage unit, stores the read out data, produces display data from stored map data and the current position data, and displays it. From the displayed information, the driver using the system can recognize the current position of the vehicle with relation to the map information.
Among navigation systems of this kind, a system has been developed which represents visual guide marks instructing turning corner left or right at certain transit points such as crossings on the way from a start point to a destination point. The navigation system calculates transit points from route data preset by user or calculated automatically by the system on the basis of data of start point and destination point defined by user. As described, an essential object of navigation system is to give driving information, such as current position or travel direction, to the user to assist his driving. Namely, it is used and operated as the user is driving, and therefore the system must not obstruct or hinder user's driving action. In this regard, there has been proposed a system which provides vocal guide messages, in place of visual guide marks, for turning corners.
However, in use of the above system, if part of travel route scheduled is altered and a certain transit point is not passed, the system cannot renew transit points subsequent to the eliminated (scheduled but not passed) transit point automatically. Further, in a case where a plurality of transit points are preset closely with each other relative to measurement accuracy of current position, the system may erroneously detect other transit point which should be passed later and therefore issue an erroneous vocal guide message to turn a corner in the wrong direction. Still further, the system does not consider the actual speed of vehicle and the time required for reaching the target transit point, and therefore the timing of vocal guide message is occasionally too early or too late.